Field emission is a phenomenon which occurs when an electric field proximate the surface of an emission material marrows a width of a potential barrier existing at the surface of the emission material. This allows a quantum tunnelling effect to occur, whereby electrons cross through the potential barrier and are emitted from the material. This is as opposed to thermionic emission, whereby thermal energy within an emission material is sufficient to eject electrons from the material. Thermionic emission is a classical phenomenon, whereas filed emission is a quantum mechanical phenomenon.
The field strength required to initiate field emission of electrons from the surface of a particular material depends upon that material's effective "work function." Many materials have a positive work function and thus require a relatively intense electric field to bring about field emission. Some materials do, in fact, have a low work function, or even a negative electron affinity, and thus do not require intense fields for emission to occur. Such materials may be deposited as a thin film onto a conductor, resulting in a cathode with a relatively low threshold voltage required to produce electron emissions.
In prior art devices, it was desirable to enhance field emission of electrons by providing for a cathode geometry which focussed electron emission at a single, relatively sharp point at a tip of a conical cathode (called a micro-tip cathode). These micro-tip cathodes, in conjunction with extraction grids proximate the cathodes, have been in use for years in field emission displays.
For example, U.S. Pat. No. 4,857,799, which issued on Aug. 15, 1989, to Spindt et al., is directed to a matrix-addressed flat panel display using field emission cathodes. The cathodes are incorporated into the display backing structure, and energize corresponding cathodoluminescent areas on a face plate. The face plate is spaced 40 microns from the cathode arrangement in the preferred embodiment, and a vacuum is provided in the space between the plate and cathodes. Spacers in the form of legs interspersed among the pixels maintain the spacing, and electrical connections for the bases of the cathodes are diffused sections through the backing structure. Spindt et al. employ a plurality of micro-tip field emission cathodes in a matrix arrangement, the tips of the cathodes aligned with apertures in an extraction grid over the cathodes. With the addition of an anode over the extraction rid, the display described in Spindt et al. is a triode (three terminal) display.
Unfortunately, micro-tips employ a structure which is difficult to manufacture, since the micro-tips have fine geometries. Unless the micro-tips have a consistent geometry throughout the display, variations in emission from tip to tip will occur, resulting in unevenness in illumination of the display. Furthermore, since manufacturing tolerances are relatively tight, such micro-tip displays are expensive to make.
For years, others have directed substantial effort toward solving the problem of creating cathodes which can be mass manufactured to tight tolerances, allowing them to perform with accuracy and precision. Another object of some of these prior art inventions was that they made use of emission materials having a relatively low effective work function so as to minimize extraction field strength.
One such effort is documented in U.S. Pat. No. 3,947,716, which issued on Mar. 30, 1976, to Fraser, Jr. et al., directed to a field emission tip on which a metal adsorbent has been selectively deposited. In a vacuum, a clean field emission tip is subjected to heating pulses in the presence of an electrostatic field to create thermal field build up of a selected plane. Emission patterns from this selected plane are observed, and the process of heating the tip within the electrostatic field is repeated until emission is observed from the desired plane. The adsorbent is then evaporated onto the tip. The tip constructed by this process is selectively faceted with the emitting planar surface having a reduced work function and the non-emitting planar surface as having an increased work function. A metal adsorbent deposited on the tip so prepared results in afield emitter tip having substantially improved emission characteristics. Unfortunately, as previously mentioned, such micro-tip cathodes are expensive to produce due to their fine geometries. Also, since emission occurs from a relatively sharp tip, emission is still somewhat inconsistent from one cathode to another. Such disadvantages become intolerable when many cathodes are employed in great numbers such as in a flat panel display for a computer.
As is evident in the above-described cathode structure, an important attribute of good cathode design is to minimize the work function of the material constituting the cathode. In fact, some substances such as alkali metals and elemental carbon in the form of diamond crystals display a low effective work function. Many inventions have been directed to finding suitable geometries for cathodes employing negative electron affinity substances as a coating for the cathode.
For instance, U.S. Pat. No. 3,970,887, which issued on Jul. 20, 1976, to Smith et al., is directed to a microminiature field emission electron source and method of manufacturing the same wherein a single crystal semiconductor substrate is processed in accordance with known integrated microelectronic circuit techniques to produce a plurality of integral, single crystal semiconductor raised field emitter tips at desired field emission cathode sites on the surface of a substrate in a manner such that the field emitters tips are integral with the single crystal semiconductor substrate. An insulating layer and overlaying conductive layer may be formed in the order named over the semiconductor substrate and provided with openings at the field emission locations to form micro-anode structures for the field emitter tip. By initially appropriately doping the semiconductor substrate to provide opposite conductivity-type regions at each of the field emission locations and appropriately forming the conductive layer, electrical isolation between the several field emission locations can be obtained. Smith et al. call for a sharply-tipped cathode. Thus, the cathode disclosed in Smith et al. is subject to the same disadvantages as Fraser, Jr. et al.
U.S. Pat. No. 4,307,507, which issued on Dec. 29, 1981, to Gray et al., is directed to a method of manufacturing a field-emitter array cathode structure in which a substrate of single crystal material is selectively masked such that the unmasked areas define islands on the underlying substrate. The single crystal material under the unmasked areas is orientation-dependent etched to form an array of holes whose sides intersect at a crystal graphically sharp point.
U.S. Pat. No. 4,685,996, which issued on Aug. 11, 1987, to Busta et al., is also directed to a method of making a field emitter and includes an anisotropically etched single crystal silicon substrate to form at least one funnel-shaped protrusion on the substrate. The method of manufacturing disclosed in Busta et al. provides for a sharp-tipped cathode.
Sharp-tipped cathodes are further described in U.S. Pat. No. 4,885,636, which issued on Aug. 8, 1989, to Busta et al.
Yet another sharp-tipped emission cathode is disclosed in U.S. Pat. No. 4,964,946, which issued on Oct. 23, 1990, to Gray et al. Gray et al. disclose a process for fabricating soft-aligned field emitter arrays using a soft-leveling planarization technique, e.g. a spin-on process.
Even through they employ low effective work-function materials to advantage, sharp-tipped cathodes have fundamental problems when employed in a flat panel graphic display environment, as briefly mentioned above. First, they are relatively expensive to manufacture. Second, they are hard to manufacture with great consistency. That is, electron emission from sharp-tipped cathodes occurs at the tip. Therefore, the tip must be manufactured with extreme accuracy such that, in a matrix of adjacent cathodes, some cathodes do not emit electrons more efficiently than others, thereby creating an uneven visual display. In other words, the manufacturing of cathodes must be made more reliable so as to minimize the problem of inconsistencies in brightness in the display along its surface.
In Ser. No. 07/851,701, which was filed on Mar. 16, 1992, and entitled "Flat Panel Display Based on Diamond Thin Films," an alternative cathode structure was first disclosed. Ser. No. 07/851,701 discloses a cathode having a relatively flat emission surface as opposed to the aforementioned micro-tip configuration. The cathode, in its preferred embodiment, employs a field emission material having a relatively low effective work function. The material is deposited over a conductive layer and forms a plurality of emission sites, each of which can field-emit electrons in the presence of a relatively low intensity electric field.
Flat cathodes are much less expensive and difficult to produce in quantity because the fine, micro-tip geometry has been eliminated. The advantages of the flat cathode structure was discussed at length therein. The entirety of Ser. No. 07/851,701, which is commonly assigned with the present invention, is incorporated herein by reference.
A relatively recent development in the field of materials science has been the discovery of amorphic diamond. The structure and characteristics of amorphic diamond are discussed at length in "Thin-Film Diamond," published in the Texan Journal of Science, vol. 41, no. 4, 1989, by C. Collins et al. Collins et al. describe a method of producing amorphic diamond film by a laser deposition technique. As described therein, amorphic diamond comprises a plurality of micro-crystallites, each of which has a particular structure dependent upon the method of preparation of the film. The manner in which these micro-crystallites are formed and their particular properties are not entirely understood.
Diamond has a negative election affinity. That is, only a relatively low electric field is required to distort the potential barrier present at the surface of diamond. Thus, diamond is a very desirable material for use in conjunction with field emission cathodes. In fact, the prior art has employed crystalline diamond films to advantage as an emission surface on micro-tip cathodes.
In "Enhanced Cold-Cathode Emission Using Composite Resin-Carbon Coatings," published by S. Bajic and R. V. Latham from the department of electronic Engineering and Applied Physics, Aston University, Aston Triangle, Burmingham B4 7ET, United Kingdom, received May 29, 1987, a new type of composite resin-carbon field-emitting cathode is described which is found to switch on at applied fields as low as approximately 1.5 MV m.sup.-1, and subsequently has a reversible I-V characteristic with stable emission currents of &gt; or =1 mA at moderate applied fields of typically &lt; or =8 MV m.sup.-1. A direct electron emission imaging technique has shown that the total externally recorded current stems from a high density of individual emission sites randomly distributed over the cathode surface. The observed characteristics have been qualitatively explained by a new hot-electron emission mechanism involving a two-stage switch-on process associated with a metal-insulator-metal-insulator-vacuum (MIMIV) emitting regime. However, the mixing of the graphite powder into a resin compound results in larger grains, which results in fewer emission sites since the number of particles per unit area is small. It is preferred that a larger amount of sites be produced to produce a more uniform brightness from a low voltage source.
In "Cold Field Emission From CVD Diamond Films Observed In Emission Electron Microscopy," published by C. Wang, A. Garcia, D. C. Ingram, M. Lake and M. E. Kordesch from the Department of Physics and Astronomy and the condensed Matter and Surface Science Program at Ohio University, Athens, Ohio on Jun. 10, 1991, there is described thick chemical vapor deposited "CVD" polycrystalline diamond films having been observed to emit electrons with an intensity sufficient to form an image in the accelerating field of an emission microscope without external excitation. The individual crystallites are of the order of 1-10 microns. The CVD process requires 800.degree. C. for the depositing of the diamond film. Such a temperature would melt a glass substrate.
The prior art has failed to: (1) take advantage of the unique properties of amorphic diamond; (2) provide for field emission cathodes having a more diffused area from which field emission can occur; and (3) provide for a high enough concentration of emission sites (i.e., smaller particles or crystallites) to produce a more uniform electron emission from each cathode site, yet require a low voltage source in order to produce the required field for the electron emissions.